1. Field of the Invention
The present invention relates to a protection device for protecting an assembled cell including a plurality of secondary batteries connected in series, and a battery pack unit incorporating a protection device.
2. Description of the Related Art
A high energy density secondary battery has been widely used as a power supply for small information equipment such as a portable phone and a notebook personal computer. The secondary batteries are often connected in series, the number of which corresponds to a voltage and current necessary for an objective apparatus or often used as an assembled cell in which they are connected in series or in parallel. Because a power supply voltage of the aforementioned small information equipment is about several V to 10 V, the number of series connections in the assembled cell is 1 to 3.
On the other hand, in recent years, application of the secondary battery to purposes requiring a high output and high voltage such as household electric appliances, power tools, assisted bicycles and hybrid electric vehicles has been expanding rapidly as well as the power supply for the information equipment. Accompanied by this, the number of series connections in the assembled cell is increased, and it is not rare that 10 or more batteries are connected in series.
A prominent problem in connecting the batteries in series is a fluctuation between individual batteries (called cells). The fluctuation includes, for example, fluctuation in capacity, fluctuation in impedance, and fluctuation in the state of charge (SOC). A fluctuation which likely leads to an error of these ones is a fluctuation in voltage which is one of the fluctuations in the SOC.
If batteries having different capacities are connected in series or a plurality of batteries are connected under different SOCs, a cell having a higher voltage or a cell having a lower voltage than the average is generated in a fully charged state of the assembled cell. The cell having a higher voltage is turned into an overcharged state, whereby deterioration is intensified. If the charge is repeated, the cell whose deterioration is intensified by the overcharge has a reduced capacity, so that the overcharge is progressed, thereby accelerating the deterioration. As a result, the cycle service life of the assembled battery is shortened extremely than the service life of the cell.
The cause for the fluctuation in the SOC includes not only the fluctuation of the cell in an initial period but also a factor generated during use of the assembled battery. For example, sometime there is a difference in temperature among the cells or currents discharged from each cell are different, thereby leading to a cause for the fluctuation in the SOC. Particularly, because the protection device is complicated in the assembled cell which has many series connections to output a high voltage, the discharge current by the protection device is apt to fluctuate.
In the assembled cell composed of a large number of batteries connected in series with a high voltage output, the protection device is generally constituted of protection units provided for each battery module each having several cells to ten and several cells for the reason of restriction about withstand voltage of an integrated circuit (IC) for use in the protection device. Each protection unit is operated using a battery module as a power supply, measures the voltages of each cells and temperature in a corresponding battery module and communicates with protection units on the upper level and lower level in order to monitor the entire system.
On the other hand, JP-A 2004-282960 (KOKAI) has disclosed a technique for correcting a fluctuation in voltage due to a difference in consumption current among blocks if an assembled cell is divided to a plurality of blocks along a voltage detecting line by an incoming current of the voltage detecting line. More specifically, a circuit or element calculated preliminarily so that consumption currents of other blocks are matched with a consumption current of the block having a maximum current value in the blocks of the assembled cell is inserted in between a cathode and an anode of the other blocks so as to correct the fluctuation in the voltage.
In the manner provided with the protection units for each module of the assembled cell, a difference in consumption current of the protection unit itself is generated due to a fluctuation of an electric circuit in the protection unit or a fluctuation in communication data amount. If there is a difference in consumption current among the protection units, a fluctuation in the SOC occurs among the modules, which is a problem to be solved.
On the other hand, although the technique of JP-A 2004-282960 (KOKAI) can correct the fluctuation in the voltage among the blocks in the assembled cell due to the incoming current for voltage detection, still the SOC fluctuation occurs because a difference in consumption current among the units due to the operation of the protection unit itself is not considered.